1. Field of the Invention
The present invention relates to an optical communication system, and more particularly to an optical communication system for transmitting optical signals of a plurality of different wavelengths by wavelength-multiplexing.
2. Description of the Related Art
A wavelength-multiplexing optical communication method is arranged to transmit optical signals of different wavelengths through a single transmission line, thereby improving a utilization factor of the transmission line. A system for the method usually includes a plurality of stations for transmitting the optical signals of different wavelengths. The system also includes light superimposing means for guiding these optical signals into the single transmission line, as well as a single transmission line, and wavelength separating means for separating only a signal of a necessary wavelength from a wavelength-multiplexed signal. The system also includes a plurality of stations receiving the multiplexed signal.
The system of this type mainly uses a semiconductor laser as a light source provided in each station and an optical fiber as the transmission line. The system also uses an optical multiplexing element composed of a half mirror, and light waveguides as the light superimposing means. The system also includes an optical wavelength filter as the wavelength separating means.
The semiconductor laser as a light source, however, has such a drawback that the wavelength of emitted light therefrom easily changes depending upon factors including temperature, etc. Thus, it has the following problems. First, spacing between the wavelengths must be increased to avoid interference. This makes it difficult to raise multiplicity. Second, introduction of wavelength reference and precise temperature control is necessary for precise wavelength control, which makes the system complicated.
In order to solve these problems, communication methods have been proposed. For example, one method is described in Japanese Laid-Open Patent Application No. 3-214830.
FIG. 5 is a schematic drawing to show a system for achieving the wavelength-multiplexing optical communication method of the conventional example. In the drawing, reference numeral 301 designates an optical star coupler, 302-1 to 302-n designate optical fibers serving as optical transmission lines, and 303-1 and 303-n designate terminal stations for optical communication.
FIG. 1 is a schematic drawing of an optical transmitter-receiver portion in each terminal station 303-1 to 303-n as shown in FIG. 5. In FIG. 1, reference numeral 2102 denotes a wavelength-variable light source, such as a semiconductor laser, capable of changing an oscillation wavelength by control from the outside. Reference numeral 2103 denotes an optical dividing/power multiplexing element for dividing signal light from the wavelength-variable light source 2102 into beams guided to the transmission line and to an optical dividing element 2107. Optical dividing/power multiplexing element 2103 also transmits the wavelength-multiplexed light from the transmission line to the optical dividing element 2107. As discussed, reference numeral 2107 denotes the optical dividing element for distributing the light from the optical dividing/power multiplexing element to wavelength-variable filters 2106-1 to 2106-3. Reference numeral 2104 denotes a circuit for extracting necessary information from photodetectors 2105-2 and 2105-3. Reference numeral 2101 denotes a control circuit for sending or receiving data to or from terminal equipment and for controlling the wavelength-variable light source 2102 and wavelength-variable filters 2106-1 to 2106-3 to avoid interference with another station. Reference numerals 2106-1 to 2106-3 denote the wavelength-variable filters whose wavelength regions of transmitted light can be changed by external control. Reference numerals 2105-1 to 2105-3 denote the photodetectors in FIG. 1.
FIG. 2 is a drawing to show a relative relation among passing wavelengths of the wavelength-variable filters 2106-1 to 2106-3 as shown in FIG. 1. In the drawing, 2201 to 2203 indicate wavelength transmitting or passing characteristics of the respective wavelength-variable filters 2106-1 to 2106-3.
These wavelength-variable filters are arranged in such a manner that when external control changes the passing wavelengths, the three passing characteristics simultaneously change by the same wavelength and in the same direction while maintaining the relative relation among the passing characteristics.
Next, the operation of the conventional example having the above arrangement will be explained. For simplicity, let us use an example where communication is made using light of wavelength .lambda.1 from the terminal station 303-1 to the terminal station 303-2 in FIG. 5 and light of wavelength .lambda.2 from the terminal station 303-3 to the terminal station 303-n.
Let us suppose herein that the wavelength .lambda.1 and wavelength .lambda.2 are proximal to each other but apart more than the wavelength width necessary for communication from each other, thus not causing interference.
In the optical transmitter-receiver in the terminal station 303-1 shown in FIG. 5, the optical dividing/power multiplexing element 2103 sends part of the signal light of wavelength .lambda.1 from the wavelength-variable light source 2102 out to the transmission line to be transmitted to the terminal station 303-2. The rest is transmitted to the optical dividing element 2107 to be divided into three beams, which reach the wavelength-variable filters #1 (2106-1), #2 (2106-2), #3 (2106-3). The wavelength-variable filter #1 (2106-1) is controlled by a control signal from the control circuit 2101 so that the center of the passing wavelength thereof may be coincident with the wavelength .lambda.1. Thus, the photodetector #1 (2105-1) supplies a large output. The photodetectors #2 (2105-2), #3 (2105-3) supply outputs associated with response amplitudes to the wavelength .lambda.1, of the wavelength-variable filters #2 (2105-2), and #3 (2105-3).
In the optical transmitter-receiver in the terminal station 303-2 on the other hand, light of the wavelengths .lambda.1, .lambda.2 coming through the transmission line passes the optical dividing/power multiplexing element 2103 and the optical dividing element 2107 to reach the wavelength-variable filters #1 (2106-1), #2 (2106-2), #3 (2106-3). Since the wavelength-variable filter #1 (2106-1) is controlled so that the center of the passing wavelength thereof may match with the wavelength .lambda.1, the light of wavelength .lambda.2 is interrupted here, and only the light of wavelength .lambda.1 is converted into an electric signal by the photodetector #1 (2106-1). Then the electric signal is transmitted through the control circuit 2101 to the terminal equipment.
As described above, the semiconductor laser is used as the wavelength-variable light source 2102, and the oscillation wavelength thereof easily changes depending upon the temperature.
The operation in the case where the wavelength changes so that the signal of wavelength .lambda.2 sent from the terminal station 303-3 approaches the wavelength .lambda.1 is explained next.
When the light of wavelength .lambda.2 comes into the passing band of the wavelength-variable filter #2 (2106-2), the output from the photodetector #2 (2105-2) increases. In contrast with it, the output from the photodetector #3 (2105-3) shows no change. Thus, an adjacent channel approach detection circuit 2104 can detect approach of light of a wavelength shorter than the wavelength .lambda.1 by checking the outputs from the two photodetectors. The adjacent channel approach detection circuit 2104 transfers the detected information to the control circuit 2101.
In accordance with the detected information, the control circuit 2101 uses a control signal of passing wavelength to effect such control as to continuously move the wavelength of the wavelength-variable light source 2102 to longer wavelengths than .lambda.1, and thereby to avoid interference of the signal with the wavelength .lambda.2 approaching. At the same time, the control circuit 2101 performs such control that the wavelength .lambda.1 may become coincident with the passing center wavelength of the wavelength-variable filter #1 (2106-1), using control signals of passing wavelengths of the wavelength-variable filters #1 (2106-1), #2 (2106-2), #3 (2106-3).
In response to the terminal station 303-1 moving the wavelength .lambda.1 to avoid interference, the terminal station 303-2 receiving the signal of wavelength .lambda.1 experiences a decrease in the output signal from the photodetector thereof #1 (2105-1). Then, using a control signal of a passing wavelength, the control circuit 2101 in the terminal station 303-2 controls the center of the passing wavelength of the wavelength-variable filter #1 (2106-1) to always maximize the output signal from the photodetector #1 (2105-1). This operation permits the terminal station 303-1 to prevent interference when the output wavelength from the terminal station 303-3 changes to approach the output wavelength of the terminal station 303-1. At the same time, the terminal station 303-2 can continuously receive the signal from the terminal station 303-1 without being tuned out.
The above function is also effective where the wavelength .lambda.2 approaches the wavelength .lambda.1 from the longer wavelength region, where only .lambda.1 changes while .lambda.2 is fixed, and where both .lambda.1, .lambda.2 change. In each of these cases, communication can continuously be maintained to avoid interference.
In the method discussed above, the signals from the respective stations are scattered on the wavelength axis to ensure they will not interfere with each other. This method, however, provides no positive device to increase the wavelength multiplicity to improve the utilization factor of the usable wavelength region.
Further, the arrangement using the wavelength filters required three wavelength-variable band-pass filters upon transmission.
Further, U.S. Pat. No. 4,592,043 discloses performing frequency scanning using a resonator to find an adjacent channel and servo-controlling a transmission frequency such that this frequency is higher than the adjacent lower channel by a predetermined channel spacing.
Further, U.S. Pat. No. 4,800,555 discloses dither-controlling a transmission wavelength to detect errors due to interference with wavelengths on longer and shorter wavelength sides of the transmission wavelength, comparing these errors with each other, and re-arranging the transmission wavelength using this comparison result as a feedback signal to reduce the errors. However, such technology is not directed to a technology for arranging wavelengths with a predetermined spacing.
In addition, EPA No. 381,102 discloses arranging channels with a predetermined frequency spacing. The technology disclosed in EPA 381,102, however, is for detecting the channel using heterodyne detection. Hence, it is impossible for this technology to determine if a wavelength adjacent to a given wavelength is on a longer wavelength side or a shorter wavelength side. U.S. Pat. No. 5,301,052 also discloses similar technology.